1. Field of the Invention
The present invention relates to a homogeneitycorrecting block for adjusting and correcting the magnetic field produced by a magnet. The invention also provides a magnet equipped with blocks of this type. A primary application of this invention is in the medical field in which magnets are employed for performing noninvasive investigations by nuclear magnetic resonance (NMR) imaging techniques. However, the invention has potential applications in other areas of activity, particularly for scientific research in which intense fields produced by magnets are employed.
2. Description of the Prior Art
Magnetic resonance is a phenomenon of oscillation of the magnetic moment of nuclei of atoms or molecules of a body at a frequency depending on the intensity of a magnetic field in which the body is placed. It is accordingly apparent that, if the intensity of the magnetic field varies, the frequency of the resonance phenomenon also varies. In experimentations of this type, it is consequently of primary importance for both technological and technical reasons to ensure that the field produced by the magnet is very homogeneous in its zone of interest. The requisite degree of homogeneity is commonly of the order of a few parts per millions in the medical field or even a few parts per billions in the scientific field. In order to achieve this objective, it is endeavored to construct magnets which produce a field having perfect homogeneity.
Unfortunately, no matter how much care may be devoted to the construction of magnets, the final products are never as perfect as the theory which has led to their design. Furthermore, even if this shortcoming can be completely overcome, the magnet still has to be materially placed at a predetermined location in order to be put to use. There is, however, no region in the earth's atmosphere either in an industrial or in an urban environment which is totally free from disturbing magnetic influences. The result is that, once a magnet is installed at its site location, the field produced in the zone of interest of the magnet exhibits inhomogeneities and an effort then has to be made to correct them.
The principle involved in correction of field inhomogeneities is that of superposition: coils, magnetic parts or any other means for producing magnetic fields are added in order to correct imperfections of the main field and in order to produce a homogeneous total field. In an article by Messrs. D.I. Hoult and D. Lee entitled "Shimming a superconducting nuclear-magneticresonance imaging magnet with steel" and published in the January 1985 issue of Rev. Sci. Instrum., pages 131 to 135, a method is described for correcting the magnetic field produced by a magnet. In accordance with this method, magnetizable soft iron bars are placed in such a manner as to exert their influence in a zone of interest of the magnet. The degree of this influence depends on the size of the bars: as their cross-section is larger, so the efficiency of correction is higher. However, this method suffers from a drawback: the magnetic parts employed have to be cut in accordance with strict dimensional requirements for each application. Furthermore, taking into account the complexity of the correction theory, the authors lay down as a postulate for the purpose of simplifying their solution that the magnetic elements can be placed only at predetermined locations with either radial or annular orientations within the magnet. As a result, correction of the field produced by a magnet cannot be performed either readily or satisfactorily unless correcting bars are manufactured and positioned in accordance with requirements.